The plunger type electromagnet is designed
1) to make use of electromagnetic attractive force acting on a movable element upon energization of a coil wound around a stationary element of magnetic substance.
Also,
2) there has been used the so-called "latching type" electromagnet wherein the magnetomotive force generated by energization of a coil and the magnetomotive force by a permanent magnet are allowed to act in series on the plunger of magnetic substance.
The above-mentioned plunger type electromagnet, however, suffers from the following disadvantages.
(1) It inherently requires the presence of a gap between a yoke and the plunger, so that a large ampere-turn is required to magnetize across such a gap. Particularly, the latching type electromagnet requires a larger ampere-turn because a permanent magnet having a large magnetic reluctance is inserted in series in the magnetic circuit developed by coil energization. This entails to enlarge the size of the electromagnet.
(2) There is another disadvantage in that the magnetic attractive force at the gap acts in a given direction along the circumference of the plunger because of the fluctuation in the magnetic flux density as viewed in the circumferential direction of the plunger, whereby the operating frictional resistance of the plunger is increased.
(3) In combination with the condition as set forth in (1) above, in the case of an electromagnet of the type in which the coil must be kept energized as long as the attractive force is to be applied, power consumption is increased accordingly.
(4) Due to deviation in the machining accuracy during mass production of electromagnets, in the material property or in the spring force and the like, there is a likelihood that under the action of the residual magnetic flux, the plunger is not released away from the stationary element even after electric current to the coil is cut off.
(5) With respect to the electromagnet of the type providing the latching function in which the plunger is retained under the action of a permanent magnet even after the power supply to the coil is cut off, there is a need for an electromagnet wherein such a permanent magnet is omitted in order to reduce the production cost, as long as the same latching function is performed in the absence of a permanent magnet.
(6) In the conventional electromagnet, the differential coefficient of the permeance, at the moment where the plunger is attracted toward the stationary element, as differentiated along the direction of movement of the plunger is so small that it is unable to obtain a relatively large initial attractive force.
(7) In the latching type electromagnet in which an annular magnet is employed as a permanent magnet, it has customarily been necessary to magnetize the annular permanent magnet in the radial direction thereof. Magnetization of the annular permanent magnet in such a direction is difficult because of large difference in surface area between the outer and inner peripheries of the annular magnet. For this reason, it has been necessary to divide the annulus into a plurality of sectoral segments. This has resulted in a poor volumetric ratio of the annular permanent magnet, bulky size of the electromagnet, increase in the number of component parts, and low productivity.